1. Field of the Invention
The present invention relates to a lithium secondary battery, and in particular relates to a lithium secondary battery using lithiummanganate as positive electrode active material and amorphous carbon material as negative electrode active material.
2. Description of the Related Art
Conventionally, in a field of a rechargeable secondary battery, an aqueous solution type battery such as a lead-acid battery, a nickel-cadmium battery, a nickel-hydrogen battery and the like was in a main trend. In recent years, however, in view of such problems as global warming and exhaustible fuel, attention has been paid to an electric vehicle (EV) and a hybrid electric vehicle (HEV) whose driving force is assisted with an electric motor, and a secondary battery with higher capacity and higher power (output) for such vehicles has been required. As a power source to meet such a need, a non-aqueous solution type lithium secondary battery which has high voltage has lately drawn attention.
Carbon material which lithium ions can be inserted in/departed from (occluded in/released from) is generally used as negative electrode material (negative electrode active material) for the lithium secondary battery. As such carbon material, for example, graphite system material such as natural graphite, scale-shaped or massive-shaped artificial graphite, mesophase pitch system; graphite or the like, or amorphous (noncrystalline) carbon material prepared by calcinating such furan resin as furfuryl alcohol or the like can be listed up. In the graphite system material, while there are advantages in that irreversible capacity is small, voltage characteristic is flat and capacity is high, but there is a disadvantage in that cycle characteristic is inferior. Also, in the amorphous carbon prepared by calcinating the synthetic resin, while there are advantages in that a capacity value exceeding a theoretical capacity value of the graphite can be obtained and the cycle characteristic is superior, but there are disadvantages in that the irreversible capacity is large and it is difficult to increase battery capacity.
Meanwhile, lithium transition metallic oxide is used as positive electrode material (positive electrode active material) for the lithium secondary battery. As the positive electrode material, lithium cobaltate is generally used in view of balances of capacity, cycle characteristic and the like. In a secondary battery using lithium cobaltate for the positive electrode material, since the quantity of cobalt resources as its raw material is small and the cobalt is costly, lithium manganate has been regarded as promising material for the EV or HEV battery, and the development has been advanced for the battery.
However, in the battery using the lithium manganate as the positive electrode material, since the lithium manganate causes elution at a high temperature of 50xc2x0 C. or so, the battery is inferior to the battery using the lithium cobaltate for the positive electrode material in cycle characteristic under the high temperature. Thus, there is a drawback in a case in which the lithium manganate is assumed to be applied to the EV or HEV. In order to overcome the drawback, there have been various proposals that manganese site of the lithium manganate is replaced with dissimilar metal such as cobalt (Co), chromium (Cr) or the like so as to decrease the manganese elution even under the high temperature and to improve the high temperature characteristic of the battery.
In the lithium manganate whose manganese site is replaced with the dissimilar metal, the manganese elution amount at the high temperature is decreased definitely, but there are drawbacks in that the manganese elution into the electrolytic solution is not only prevented completely but also discharge capacity is decreased.
The present inventors have studied and analyzed the causes of the cycle deterioration at the high temperature in the battery using the lithium manganate as the positive electrode material and the amorphous carbon material as the negative electrode material. As a result, the inventors have found out that the cycle deterioration at the high temperature is caused by formation of inert coating on a surface of the negative electrode because of the manganese eluted from the positive electrode acting as cores of the inert coating.
In view of the above drawbacks and based upon the findings, a first object of the present invention is to provide a lithium secondary battery capable of improving cycle characteristic effectively without decreasing discharge capacity.
A second object of the invention is to provide a lithium secondary battery capable of improving charging/discharging cycle life and preservation life under a high temperature.
In order to achieve the first object, a first aspect of the present invention is a lithium secondary battery, comprising a positive electrode having a positive electrode collector to which mixture containing lithium manganate as positive electrode active material is applied; and a negative electrode having a negative electrode collector to which mixture containing amorphous carbon material as negative electrode active material is applied, wherein a mean particle diameter of the amorphous carbon material is 10 xcexcm or less. In this aspect, since the mean particle diameter of the amorphous carbon material is made to be 10 xcexcm or less, surface area of the amorphous carbon material becomes large. Therefore, even when the inert coating is formed on the surface of the negative electrode due to the manganese elution from the positive electrode/the manganese deposition on the negative electrode, as a total surface area of the amorphous carbon material is large, the high temperature cycle characteristic of the secondary battery can be improved without decreasing discharge capacity.
In this aspect, the specific surface area of the amorphous carbon material having the mean particle diameter of 10 xcexcm is about 5 m2/g, and when the specific surface area is less than 5 m2/g, an effect of a surface area increase is hardly obtained. The specific surface area of the amorphous carbon material with the mean particle diameter of 3.5 xcexcm is about 20 m2/g, and when the specific surface area is 20 m2/g or more, the specific surface area is made excessively large so that deterioration in other performances such as an irreversible capacity increase and the like occurs. Therefore, it is preferable that the mean particle diameter of the amorphous carbon material is in the range of 3.5 xcexcm or more and 10 xcexcm or less. Further, when a Li/Mn ratio in the lithium manganate is in a range of more than 0.5 and 0.6 or less, a manganese elution amount can be reduced without decreasing the discharge capacity extremely as compared with the reduction in a case of the stoichiometric composition (0.5).
In order to achieve the second object, a second aspect of the invention is a lithium secondary battery, comprising a positive electrode having a positive electrode collector to which mixture containing lithium manganate as positive electrode active material is applied; and a negative electrode having a negative electrode collector to which mixture containing amorphous carbon material as negative electrode active material is applied, wherein irreversible capacity of the amorphous carbon material is in a range of 5% or more and 25% or less of initial charge capacity, and a discharge capacity ratio (xe2x88x92/+ ratio) of the negative electrode to the positive electrode after the initial charge is in a range of 1.3 or more and 1.8 or less. In this aspect, since the depth of discharge in the positive electrode becomes small as much as the irreversible capacity of the negative electrode increases by making the amount of the negative electrode active material in the lithium secondary: battery excessive so as to make the discharge capacity ratio of the negative electrode to the positive electrode large, deterioration of the positive electrode can be suppressed. Since the utilization factor of the negative electrode also becomes small due to the excess in the negative electrode active material, deterioration of the negative electrode can be suppressed. When the xe2x88x92/+ ratio is less than 1.3, an effect obtained by increasing the ratio is small, and when the xe2x88x92/+ ratio exceeds 1.8, as the load of the positive electrode becomes large and the battery capacity is reduced in spite of increasing the ratio. Accordingly, it is necessary to set the xe2x88x92/+ ratio in the range of at least 1.3 and at most 1.8. According to the present invention, since the deterioration of the positive and negative electrodes can be suppressed, the charging/discharging cycle life and preservation life can be improved.
In this aspect, when a Li/Mn ratio in the lithium manganate is set to at least 0.55 and at most 0.6, the amount of manganese elution can be reduced without decreasing the battery capacity extremely as compared with the stoichiometric composition (0.5). Thus, the above range is desirable for improvement in discharge cycle life and preservation life even under the high temperature.
The present invention will become more obvious with reference to the following preferred embodiments.
(First Embodiment)
A first embodiment where the present invention is applied to a cylindrical lithium secondary battery for a vehicle will be explained hereinafter. First, manufacturing procedure of the cylindrical lithium secondary battery according to the present embodiment will be described in order of a negative electrode, a positive electrode and assembly of the battery.
 less than Negative Electrode greater than 
90 weight parts of amorphous carbon powder serving as negative electrode active material having a mean particle diameter of 3.5 xcexcm to 10 xcexcm and a predetermined specific surface area described later is added with 10 weight parts of polyvinylidene fluoride (PVDF) as binder, and it is added and mixed with N-methylpyrrolidone as dispersion solvent to produce slurry. The slurry thus obtained is applied to both surfaces of a rolled copper foil with a thickness of 10 xcexcm serving as a negative electrode collector and subsequently the rolled copper foil applied is dried. Thereafter, the negative electrode collector on which mixture layers containing the negative electrode active material are formed is pressed and then cut to obtain a negative electrode with a thickness of 70 xcexcm.
 less than Positive Electrode greater than 
100 weight parts of lithium manganate serving as positive electrode active material with a ratio of lithium to manganese (Li/Mn ratio) of more than 0.5 and at most 0.6 is added with 10 weight parts of scale-shaped graphite as electroconductive material and 5 weight parts of PVDF as binder, and it is added and mixed with N-methylpyrrolidone as dispersion solvent to produce slurry. The slurry thus obtained is applied to both surfaces of an aluminum foil serving as a positive electrode collector with a thickness of 20 xcexcm and subsequently the aluminum foil applied is dried. Thereafter, the positive electrode collector on which mixture layers containing the positive electrode active material are formed is pressed and then cut to obtain a positive electrode with a thickness of 70 xcexcm.
 less than Assembly of Battery greater than 
The negative electrode and the positive electrode thus obtained are wound with two sheets of polyethylene-made separators each having a thickness of 25 xcexcm through which lithium ions can pass and interposed therebetween to manufacture a winding group or winding body. After the winding group is inserted into a cylindrical battery container or can, a predetermined amount of electrolytic solution is poured into the battery container, and an upper opening portion of the battery container is caulked with a lid disposed inside the upper portion of the battery container so that a cylindrical lithium secondary battery is assembled. The electrolytic solution is prepared previously in the following manner. Lithium hexafluorophosphate (LiPF6) is dissolved at 1 mole/liter into mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC). The design capacity of the cylindrical lithium secondary battery is 4.0 Ah.